<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #main {
      width: 100%;
      margin: 100px auto;
      display: flex;
      flex-flow: row nowrap;
      justify-content: space-evenly;
    }

    #main > div {
      width: 500px;
      height: 300px;
      border-radius: 20px;
    }

    #main > div:first-child {
      background: antiquewhite;
      outline: 3px solid cadetblue;
      box-shadow: 0 0 0 3px cadetblue;
    }

    #main > div:last-child {
      background: brown;
      outline: 8px solid cadetblue;
      box-shadow: 0 0 0 8px cadetblue;
    }
  </style>
</head>
<body>
  <div id="main">
    <div></div>
    <div></div>
  </div>
</body>
</html>